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Coastal earthquake nothing to be shaken up about

3.3-magnitude quake 'common for the area,' Geological Survey reports

Monday morning's 3.3-magnitude earthquake off the coast of San Francisco is common for the area and nothing to be shaken up about, U.S. Geological Survey earthquake geologist David Schwartz said.

The quake, which struck at 7:47 a.m., had a depth of 4.97 miles and was centered 5.7 miles northwest of Daly City, Schwartz said.

"This is not uncommon at that location," he said. "It's one part of the San Andreas Fault that has a fair amount of small earthquake activity."

At least three aftershocks ranging in magnitude from 1.2 to 2.4 were reported shortly after the initial earthquake, according to Schwartz. He said the earthquake was associated with the San Andreas Fault, which runs west of San Francisco through the Santa Cruz Mountains, and that the quake was part of one of the smaller faults propagating out of the San Andreas Fault.

No one knows how many little faults there are in the earthquake's region between Daly City and halfway to the Golden Gate Bridge because they are underwater, according to Schwartz. He said, though, that small earthquakes such as Monday's occur several times a year. An earthquake on a similar type of fault struck Daly City in 1957 and was reported as a 5.3-magnitude quake. At the time, it was the largest earthquake to hit the Bay Area since the infamous earthquake in 1906.

Schwartz emphasized that little quakes in faults associated with the San Andreas Fault do not indicate a bigger earthquake is on its way. The latest USGS earthquake probability study, published in 2008, revealed a 63 percent chance an earthquake of 6.7 magnitude or greater could hit an urban spot in the Bay Area by the year 2036.

The quake could strike any one of the Bay Area's seven main faults, and Schwartz said it's important for residents to be prepared in the event of a major earthquake with emergency supplies and a family plan.